Up to now, a large number of inventions have been made on a spindle synchronization control for a machine tool, for example, as disclosed in Japanese Patent Unexamined Publication No. Hei 1-228750, Japanese Patent Unexamined Publication No. Hei 1-228751, Japanese Patent Unexamined Publication No. Hei 1-228752, Japanese Patent Unexamined Publication No. Hei 2-109605, etc.
All that are disclosed in those publications are inventions for making accurate synchronization by driving those two main spindles by a speed control servo motor when two spindles are decoupled from each other not through a workpiece.
In particular, the invention disclosed in Japanese Patent Unexamined Publication No. Hei 1-228750 discloses a technique by which a speed deviation between both the spindles is detected to make the speed synchronization of those two spindles. The invention disclosed in Japanese Patent Unexamined Publication No. Hei 1-228751 discloses a technique by which synchronization is made additionally using a positional deviation between both the spindles. The invention disclosed in Japanese Patent Unexamined Publication No. Hei 1-228752 discloses a technique by which a torque is employed in addition to the speed and the position. The invention disclosed in Japanese Patent Unexamined Publication No. Hei 2-109605 discloses a technique by which not only the speed of the spindles but also the position thereof is synchronized. Thus, those inventions are designed such that the spindles are driven by the speed control servo motor.
Also, the invention disclosed in Japanese Patent Unexamined Publication No. Hei 2-41693 conducts the positional control by an auxiliary spindle drive servo motor out of two spindle servo motors. However, in the invention, a command is of a position signal from a main spindle position detector, which clearly causes a lag, and both the main and auxiliary spindles are not driven by the same position command.
The reason that most of the prior art drive the spindles by the speed control servo motor as described above is that in the spindle drive servo motor for a machine tool, since there are many cases in which a rated speed is high although the maximum torque is not so large, if the mechanical coupling of both the spindles is made during a period of time until the speed reaches the rated speed since the servo motor starts, or after the speed is greatly changed, the speed control that enables a torque required for increasing or decreasing the speed to be increased or decreased by the maximum torque generated by the motor is advantageous in time.
However, what cannot accurately adjust the change gear ratio between the servo motor and the spindles is required to conduct the control of the spindle per se which serves as a reference, that is, the positional control of the spindle which is a final mechanical edge.
However, the above-described conventional synchronization control device for a servo motor suffers from problems stated below.
When the main spindle and the auxiliary spindle are coupled to each other through a workpiece both ends of which are gripped by chucks, if there is a difference in the total position gain between both the spindles, that is, the position gain pertaining to the operation of the spindles and the change gear ratio between the spindles and the servo motors (the rotating speed of the servo motors/the rotating speed of the spindles), a difference occurs between the position droop of the main and auxiliary spindles. This causes such problems that the torque of one servo motor reaches the torque limit, or if the fastening force of the chucks is weak, the workpiece is displaced and damaged.
In other words, the difference in total gain is caused because an accurate change gear ratio is not obtained in the case where a flexible structure such as a belt is employed for torque transmission between the spindles and the servo motors. Then, if there is a slight difference between the actual change gear ratio and a parameter set on amplifiers of the servo motors, a difference occurs in the above position droop in the position control. On the other hand, a speed difference occurs in case of the speed control.
For the above reason, in any cases, a phase difference occurs within the main and auxiliary servo motors which are coupled to each other, as the result of which the load torque of the servo motors is increased up to the torque limit value, or the workpiece is displaced if the fastening force of the chucks is weak, to thereby damage the workpiece.
In particular, as the representative application method of the spindle synchronization, a work for supporting both ends of the workpiece and cutting a center portion of the workpiece is greatly effective in acceleration of processes after cutting, and since the cutting work is conducted at a constant peripheral speed for the workpiece, the spindle synchronization control that enables the rapid acceleration/deceleration is desirable in a reduction of working time. However, the prior art does not permit even a slight displacement of the above change gear ratio.
Also, in general, since there are many cases in which the chuck of the auxiliary spindle grips the workpiece in a state where the main spindle permits the workpiece to rotate, a large force is exerted on the main and auxiliary spindles and the servo motors at that time, the rotations of the main and auxiliary spindles are lowered, resulting in the event that mechanical coupling is made in a state where the main and auxiliary spindles are displaced from a designed position. In this event, the coupling work of the auxiliary spindle must be made again, thereby leading to such a problem that troublesomeness or time is taken for working.
FIGS. 16 and 17 show a change in a difference of the position droop before and after both of the main and auxiliary spindles are coupled to each other through the workpiece, and a change in a difference of the position droop before and after both of the main and auxiliary spindles are coupled to each other not through the workpiece.
FIG. 17 showing a change in the difference of the position droop when there is no workpiece shows an appearance in which when a work of fastening the chuck on the auxiliary spindle side is made, an extremely large force is exerted on the servo motor on the auxiliary spindle side, with the results that the speed is decreased and the difference of the position droop becomes temporarily large but becomes soon small.
However, in the case where the workpiece is held by the chuck on the main spindle side and a work of fastening the chuck on the auxiliary spindle side is made, coupling may be made at the time when the speed decreases. If the chuck on the auxiliary spindle side is fastened to couple the main and auxiliary spindles to each other in the above manner, the difference between the position droops of the main and auxiliary spindles rises suddenly in a stepped state as shown in FIG. 16 and is continued until the position droop is released till chuck-off since chuck-on.
For that reason, since in the position control, a force that recovers a phase lag is exerted and reaches the torque limit value, it is necessary to correct the lag amount and to try coupling again.
In view of the above, the present invention has been made to solve the above problems, and therefore has an object to provide a synchronization control device for a servo motor, which enables the accurate synchronization driving of a main spindle and an auxiliary spindle and enables a rapid acceleration/deceleration driving even if a difference exists between the position droops of both the spindles when the main spindle and the auxiliary spindle are coupled to each other, and in particular, which can automatically cope with a change in a change gear ratio with a time even if a flexible structure such as a belt is employed for torque transmission between the spindles and servo motors.